PROJECT SUMMARY By 2050, 35% of US children will be Latino1, and these youth will number 28 million in the US public schools2. Latino youth are at disproportionate risk for growing up in poverty, attending economically disadvantaged schools, and being exposed to stressors associated with their minority status3;4. Yet, they are under- represented in prevention science5;6. When included in trials, Latinos have low rates of recruitment and retention. Yet evidenced-based family programs7-9 designed to enhance protective factors (interpersonal and socio-emotional competencies; positive family dynamics) can buffer the negative impacts of contextual adversity10;11. Thus, identifying efficacious programs that can successfully engage youth and parents in economically disadvantaged settings is critical for Latino youth, for their families, and for the nation. The proposed project tests the efficacy of an innovative, family-focused program?Siblings are Special (SIBS)? which focuses on sibling relationships and parenting of siblings as synergistic targets of change to promote positive interpersonal family dynamics and parent and youth psychosocial and behavioral health and well- being. This translational effort builds on strong theoretical and empirical premises including a successful pilot study12. Using a rigorous design and measurement approach, aims are to: (a) test the efficacy of SIBS, delivered via 12 weekly afterschool sibling sessions and 3 family nights in the familiar elementary school setting, versus a contact-equivalent attention control condition; and (b) test whether cultural resources (familism values), sociocultural risks, and sibling dyad characteristics (birth order, sibling gender, and sibling dyad gender constellation) moderate program efficacy. Mexican-origin sibling dyads (5th graders and younger siblings; N = 384 dyads) and parents will be recruited from economically disadvantaged elementary schools and randomly assigned within school to intervention or contact-equivalent attention control conditions. Assessments will be conducted at pre-test, post-test, and 18-month follow-up and will include: (a) interviews/surveys with parents and target siblings; (b) teacher ratings and school grades; and (c) daily diary data collected on 8 consecutive days from target siblings. The project will test program effects on short- and long-term outcomes, including siblings' socio-emotional skills and adjustment, school engagement and achievement, daily positive and negative well-being, family relationship quality, and siblings' and parents' global psychosocial well-being. The size and growth of the Latino/Mexican-origin population13, coupled with their disproportionate risk of living in economically disadvantaged settings3;4 make them an ideal target for an innovative family-based approach that applies dissemination principles to enhance recruitment and retention. Findings will advance prevention science by identifying an efficacious program that capitalizes on cultural assets to promote positive family dynamics and psychosocial well-being among Latinos, including by incorporating daily measurements of intervention targets and outcomes to identify mechanisms underlying program effects.